1. Field of Invention
The present invention relates to a dynamic bandwidth allocation method in passive optical network (PON), and more particularly, to a dynamic bandwidth allocation method with a punishment mechanism, wherein the time allocation of each path during each cycle period is variable.
2. Related Art
Recently, PON has been increasingly utilized in research and industry. PON mainly includes Ethernet PON (EPON) and broadband PON (BPON) used in asynchronous transfer mode (ATM) network and so on. The main architecture of EPON is shown in FIG. 1, mainly constituting an optical line terminal (OLT) 10, a 1×N optical splitter 12, multiple optical network units (ONU) 14, and optical fibers 16. The optical splitter 12 is mostly 1×16, 1×32, or 1×64, so the number of the ONU 14 is 16, 32, or 64 respectively. During the operation, data are transmitted from OLT 10 to ONU 14 by broadcasting, which is referred to as downloading; while data are uploaded from ONU 14 to OLT 10 by time division multiplexing (TDM). That is to say, when data are downloaded, every ONU 14 receives data and then selects and reads the related data. When data are uploaded, the OLT 10 sequentially orders specific ONUs 14 to upload data. At this time, the ordered ONUs 14 must upload data to the OLT 10 in a burst mode, which is referred to as the so-called operational mode of multipoint control protocol (MPCP) of PON.
During the PON operation, the data upload bandwidth allocation manner is mainly classified into two types: one is fixed bandwidth allocation, and the other is dynamic bandwidth allocation. The operational principle of fixed bandwidth allocation is that each ONU 14 has a fixed bandwidth, for example, 65% for upload, and 35% for download. The dynamic bandwidth allocation manner can be seen in the thesis “Dynamic Bandwidth Allocation for Quality-of-Service Over Ethernet PONs”, by Chadi M. Assi, Yinghua Ye, Sudhir Dixit, and Mohamed A. Ali, Institute of Electrical and Electronics Engineers (IEEE) Journal on Selected Areas in Communications, Vol. 21, No. 9, November 2003, pp. 1467-1477, wherein dynamic bandwidth allocation can be carried out by formulas (a)-(c):
                              B          i          g                =                  {                                                                                          R                    i                                    ,                                                                                                  if                    ⁢                                                                                  ⁢                                          R                      i                                                        ≤                                      B                    i                    MIN                                                                                                                                                                  B                      i                      MIN                                        +                                          B                      i                      excess                                                        ,                                                                                                  if                    ⁢                                                                                  ⁢                                          R                      i                                                        >                                      B                    i                    MIN                                                                                                          (        a        )                                          B          i          MIN                =                                                            (                                                      T                    cycle                                    -                                      N                    ×                    T                                                  )                            ×              r                        8                    ×                      w            i                                              (        b        )                                          B          i          excess                =                                            B              Total              excess                        ×                          R              i                                                          ∑                              k                ∈                K                                      ⁢                          R                              k                i                                                                        (        c        )            
Formula (a) is the basic bandwidth allocation method, wherein Ri represents the required bandwidth of ONUi, BiMIN represents the minimum bandwidth of ONUi, Biexcess represents excess bandwidth which is re-allocated to ONUi, and Big is the bandwidth actually obtained by ONUi. It can be seen from Formula (a) that, when the required bandwidth is smaller than the minimum bandwidth, ONUi can be allocated with the required bandwidth; otherwise, besides the minimum bandwidth, ONUi can also be allocated with the excess bandwidth. The minimum bandwidth is shown by Formula (b), wherein Tcycle represents a cycle period, N represents the number of ONU, Tg represents the switching time of ONU, r represents the transmission rate, and wi represents the minimum bandwidth weight of ONU (determined by the user agreement). The excess bandwidth is shown by Formula (c), wherein BTotalexcess represents the total number of the excess bandwidth, Ri represents the bandwidth required by ONUi, and K={Ri>BiMIN}.
Though Formulas (a) to (c) can effectively utilize the excess bandwidth, allocating excess bandwidth according to the proportion of the required bandwidth Ri of the ONUi does not satisfy the principle of fairness. As a result, users feeing for a small amount of bandwidth may be allocated with a great deal of bandwidth upon request.
Furthermore, the thesis also proposes using flow forecast and allocating excess bandwidth beforehand to reduce the waiting time of data with high priority. It mainly constitutes bandwidth requirements of high, moderate, and low priorities. The flow forecast mechanism is applied to the high priority part, and operated by the defined allocation manner, so as to reduce the average transmission delay of data with high priority. However, this method may increase the average transmission delay of data with other priorities, and the flow forecast error may reduce the utilization rate of the bandwidth.
Moreover, U.S. Pat. No. 6,826,160 provides a dynamic bandwidth allocation method, wherein the current flow situation is obtained from the ONU and the bandwidth is dynamically generated by adding in other conditions. The method can dynamically allocate bandwidth, but it may cause unfairness in that users have extremely large flow to occupy too much bandwidth. Meanwhile, this method is not particularly applied to an EPON network.
Moreover, U.S. Pat. No. 6,891,856 mainly provides a method and system for solving the delay problem. However, the bandwidth of quality of service (QoS) cannot be definitely guaranteed.
In view of the above, conventional dynamic bandwidth allocation has the following two disputed points. One is unfairness, wherein if an ONU continues to ask for a great deal of bandwidth over a long time, the ONU may continue to occupy excess bandwidth, thus causing unfair bandwidth allocation. The other is the delay problem, wherein during the network transmission, if a few ONUs continue to ask for a great deal of bandwidth over a long time, though the foregoing method can reduce their delay time, which majority of the other ONUs are prolonged, so the total delay time of the whole EPON is increased.